1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to equipment for generating optical duobinary signals.
2. Description of the Related Art
Duobinary signaling was introduced in the 1960s and since then has found numerous applications in communication systems. The principle of duobinary signaling is explained, for example, in an article by A. Lender that appeared in IEEE Transactions on Communications and Electronics, vol. 82 (May, 1963), pp. 214-218. Briefly, duobinary signaling uses three signal levels, for example, “+1”, “0”, and “−1”. A signal corresponding to one of these levels (i.e., a duobinary symbol) is transmitted during each signaling interval (duobinary bit period). A duobinary signal is typically generated from a corresponding binary signal using certain transformation rules, according to which direct transitions between the “+1” and “−1” levels are forbidden. Although both the binary signal and the corresponding duobinary signal carry the same information, the bandwidth of the duobinary signal may be reduced by a factor of 2 compared to that of the binary signal.
In optical communication systems, duobinary encoding is typically implemented using phase modulation of a carrier optical beam as disclosed in U.S. Pat. No. 5,867,534, which is incorporated herein by reference in its entirety. More specifically, for the “0” bit, substantially no light is transmitted. However, the “+1” and “−1” bits are transmitted as light having +E and −E electric fields, respectively, where the opposite polarities of the electric field represent a relative phase shift of 180 degrees.
A typical prior-art optical duobinary transmitter employs a Mach-Zehnder modulator (MZM) driven by a corresponding full-rate electrical signal. For example, a prior-art optical duobinary transmitter configured to generate a 100-Gb/s optical duobinary signal is driven by a 100-Gb/s electrical signal. However, with the currently available technology, electronic components adapted to operate at 100 Gb/s, such as electrical multiplexers and amplifiers for MZM drivers, are difficult and/or expensive to make. In addition, losses in coaxial cables and connectors tend to increase with increasing bit rates.